1. Field of Invention
The present invention relates to a display device, and in particular, to an adjustment method of a display device that does not emit light by itself, that is, a display device having a light source and an optical modulating device.
2. Description of Related Art
As display devices each using a light source for display and an optical modulating device, for example, liquid crystal display devices, electro-luminescence (EL), digital micromirror devices (DMD), and the like are known. In these display devices, for example, liquid crystal display devices, various types of adjustment shown below are performed as a process in a manufacturing phase in a production line of fabrication. Here, as an example, the above-described various types of adjustment methods in the case of displaying in a normally white mode in an active matrix liquid crystal display device using TFTs (thin film transistors) as pixel switches, and the general relationship between the optical power and the applied voltage will be described.
In general, in a TFT liquid crystal display device, one of two substrates where liquid crystal is sealed is processed as a TFT active matrix substrate, and the other substrate is processed as a counter substrate and a common electrode is formed. Furthermore, on the TFT active matrix substrate, pixel electrodes, which are connected to respective TFTs that are pixel switches, are formed. Moreover, a signal voltage is applied to this pixel electrode, a constant common voltage is applied to the common electrode on the counter substrate, and a differential voltage between the common voltage to the common electrode and the signal voltage to the pixel electrode is applied to each liquid crystal that is each pixel.
In addition, in the display method in the normally white mode, as shown in FIG. 17, the smaller the differential voltage between the signal voltage and common voltage is, the brighter the image display is, and the larger the differential voltage is, the darker the image display is. Here, in a graph of FIG. 17, the vertical axis shows the transmittance ratio T (brightness), the horizontal axis shows the voltage V (positive voltage: +, and negative voltage: -), and the graph shows the relationship between the differential voltage between these voltages and the counter substrate voltage, and the transmittance ratio. Thus, in a liquid crystal display device, if the differential voltage from the counter substrate voltage is small in the liquid crystal display device, the transmittance ratio is high and thereby the image display is bright, and if the differential voltage from the counter substrate voltage is large, the transmittance ratio is low and thereby the image display is dark.
FIG. 18 shows changes of the signal voltage in the case of driving the liquid crystal through reversing the polarity of the voltage applied to the liquid crystal. Thus, FIG. 18 shows changes of voltages that both of a picture signal 2 that has a positive polarity in the case of positive polarity driving against the common voltage 1 in the image display and a picture signal 3 that has a negative polarity in the case of negative polarity driving.
In this type of liquid crystal display device, adjustment of the common voltage to the common electrode (this is called common adjustment) exists as a first adjustment item. This is to adjust the common voltage 1 applied to the common electrode on the counter substrate shown in FIG. 18, and images without flicker on a screen can be obtained by this adjustment.
As a second adjustment item, there is contrast adjustment or dynamic-range adjustment. This is to adjust a contrast ratio by simultaneously adjusting an amplitude VA between the maximum value 2A and minimum value 2B of the voltage, which the picture signal 2 having a positive polarity has, and an amplitude VB between the minimum value 3A and maximum value 3B of the voltage, which the picture signal 3 having a negative polarity has, of those signals which are shown in FIG. 18.
As a third adjustment item, there is DC offset adjustment. This is adjustment of setting an area to which a voltage is applied in the applied-voltage vs optical power characteristic described later and which determines a screen condition given to a user, and adjustment of a DC level with keeping a voltage amplitude of the picture signal constant.
This is to adjust the voltage VC between the maximum value 2A of the voltage, which the positive-polarity picture signal 2 shown in FIG. 18 has, and common voltage 1 and an absolute value of the voltage VD between the minimum value 3 A of the voltage, which the negative-polarity picture signal 3 has, and common voltage 1 to equal offset voltages. Thus by adjusting these offset voltages while looking at a picture displayed on a liquid crystal panel or a color picture on a projector screen in case of a projector, respective color levels in the cases of positive-polarity driving and negative-polarity driving are determined simultaneously. Generally, this DC offset adjustment is performed in the state of the positive display on the liquid crystal panel, and hence this is also called white-balance adjustment.
As a fourth adjustment item, setting of a gamma correction characteristic exists. A liquid crystal display device comprises a gamma correction circuit in a data processor. As shown in FIG. 19(A), first, the inherent applied voltage (V) vs transmittance ratio (T) characteristic of the liquid crystal panel is measured. Then, the gamma correction characteristic shown in FIG. 19(B) is determined as correction data necessary for correcting this inherent applied voltage vs transmittance ratio characteristic of the liquid crystal panel to a linear characteristic shown in FIG. 19(C). Furthermore, by obtaining such a linear input/output characteristic that is shown in FIG. 19(C) with synthesizing the gamma correction characteristic shown in FIG. 19(B) with the applied voltage vs transmittance ratio characteristic obtained with luminance measurement shown in FIG. 19(A), the gamma correction is completed.
Here, when various types of adjustment described above are performed, it is necessary to apply light from a light source built in an actual product, for example, a backlight to a liquid crystal panel.
At this time, in this type of light source, as shown in FIGS. 2(A) and 2(B), brightness is not always constant. Here, although, in graphs shown in FIGS. 2(A) and 2(B), the vertical axes show the luminance (lux) in both graphs and the horizontal axes show the time in both graphs, the unit of the vertical axis in FIG. 2(A) is msec, but the unit of the vertical axis in FIG. 2(B) is sec.
Although the graphs in FIGS. 2(A) and 2(B) show the results of luminance measurement regarding different light sources as objects, luminance vs time characteristic waveforms of different light sources, as shown in FIGS. 2(A) and 2(B), are different because of manufacturing dispersion even if they have the same ratings, for example, luminance dispersion of discharge tubes such as metal halide lamps usually used as backlights is nearly 10% according to discharge states. In particular, this luminance dispersion, as seen from FIGS. 2(A) and 2(B), arises in a short time and/or a long time in some cases.
In this manner, if the optical power from a light source decreases or increases at the time of adjustment, measurement necessary for the third and forth adjustment items among the above-described adjustment items is hindered. Thus, when the DC offset adjustment and setting of the gamma correction characteristic are performed at the time when the optical power from the light source is abnormal, it becomes impossible to perform proper positive display and gamma correction at the time when the optical power from the light source is normal.